Objection 1. It would seem that matrimony is not natural. Because "the natural law is what nature has taught all animals" [Digest. I, i, de justitia et jure, 1.] But in other animals the sexes are united without matrimony. Therefore matrimony is not of natural law.
Objection 1. Further, that which is of natural law is found in all men with regard to their every state. But matrimony was not in every state of man, for as Tully says (De Inv. Rhet.), "at the beginning men were savages and then no man knew his own children, nor was he bound by any marriage tie," wherein matrimony consists. Therefore it is not natural.
Objection 3. Further, natural things are the same among all. But matrimony is not in the same way among all, since its practice varies according to the various laws. Therefore it is not natural.
Objection 4. Further, those things without which the intention of nature can be maintained would seem not to be natural. But nature intends the preservation of the species by generation which is possible without matrimony, as in the case of fornicators. Therefore matrimony is not natural.
On the contrary, At the commencement of the Digests it is stated: "The union of male and female, which we call matrimony, is of natural law."
Further, the Philosopher (Ethic. viii, 12) says that "man is an animal more inclined by nature to connubial than political society." But "man is naturally a political and gregarious animal," as the same author asserts (Polit. i, 2). Therefore he is naturally inclined to connubial union, and thus the conjugal union or matrimony is natural.
I answer that, A thing is said to be natural in two ways. First, as resulting of necessity from the principles of nature; thus upward movement is natural to fire. In this way matrimony is not natural, nor are any of those things that come to pass at the intervention or motion of the free-will. Secondly, that is said to be natural to which nature inclines although it comes to pass through the intervention of the free-will; thus acts of virtue and the virtues themselves are called natural; and in this way matrimony is natural, because natural reason inclines thereto in two ways. First, in relation to the principal end of matrimony, namely the good of the offspring. For nature intends not only the begetting of offspring, but also its education and development until it reach the perfect state of man as man, and that is the state of virtue. Hence, according to the Philosopher (Ethic. viii, 11,12), we derive three things from our parents, namely "existence," "nourishment," and "education." Now a child cannot be brought up and instructed unless it have certain and definite parents, and this would not be the case unless there were a tie between the man and a definite woman and it is in this that matrimony consists. Secondly, in relation to the secondary end of matrimony, which is the mutual services which married persons render one another in household matters. For just as natural reason dictates that men should live together, since one is not self-sufficient in all things concerning life, for which reason man is described as being naturally inclined to political society, so too among those works that are necessary for human life some are becoming to men, others to women. Wherefore nature inculcates that society of man and woman which consists in matrimony. These two reasons are given by the Philosopher (Ethic. viii, 11,12).
Reply to Objection 1. Man's nature inclines to a thing in two ways. In one way, because that thing is becoming to the generic nature, and this is common to all animals; in another way because it is becoming to the nature of the difference, whereby the human species in so far as it is rational overflows the genus; such is an act of prudence or temperance. And just as the generic nature, though one in all animals, yet is not in all in the same way, so neither does it incline in the same way in all, but in a way befitting each one. Accordingly man's nature inclines to matrimony on the part of the difference, as regards the second reason given above; wherefore the Philosopher (Ethic. viii, 11,12; Polit. i) gives this reason in men over other animals; but as regards the first reason it inclines on the part of the genus; wherefore he says that the begetting of offspring is common to all animals. Yet nature does not incline thereto in the same way in all animals; since there are animals whose offspring are able to seek food immediately after birth, or are sufficiently fed by their mother; and in these there is no tie between male and female; whereas in those whose offspring needs the support of both parents, although for a short time, there is a certain tie, as may be seen in certain birds. In man, however, since the child needs the parents' care for a long time, there is a very great tie between male and female, to which tie even the generic nature inclines.
Reply to Objection 2. The assertion of Tully may be true of some particular nation, provided we understand it as referring to the proximate beginning of that nation when it became a nation distinct from others; for that to which natural reason inclines is not realized in all things, and this statement is not universally true, since Holy Writ states that there has been matrimony from the beginning of the human race.
Reply to Objection 3. According to the Philosopher (Ethic. vii) "human nature is not unchangeable as the Divine nature is." Hence things that are of natural law vary according to the various states and conditions of men; although those which naturally pertain to things Divine nowise vary.
Reply to Objection 4. Nature intends not only being in the offspring, but also perfect being, for which matrimony is necessary, as shown above.
Source: Summa Theologica (New Advent)